The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to a carrier head for chemical mechanical polishing.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d or a fixed-abrasive pad. A standard polishing pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. Some carrier heads include a flexible membrane that provides a mounting surface for the substrate, and a retaining ring to hold the substrate beneath the mounting surface. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the substrate. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles, if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
A reoccurring problem in CMP is the so-called xe2x80x9cedge-effectxe2x80x9d, i.e., the tendency of the substrate edge to be polished at a different rate than the substrate center. The edge effect typically results in non-uniform polishing at the substrate perimeter, e.g., the outermost three to fifteen millimeters of a 200 millimeter (mm) wafer. A related problem is the so-called xe2x80x9ccenter slow effectxe2x80x9d, i.e., the tendency of the center of the substrate to be underpolished.
In one aspect, the invention is directed to a carrier head for a chemical mechanical polishing apparatus. The carrier head has a first pressurizable chamber at least partially bounded by a first flexible membrane, and a second pressurizable chamber positioned to apply a downward force to the first chamber. A lower surface of the first flexible membrane provides a first surface to apply a pressure to a substrate in a loading area having a controllable size, and the first and second chambers are configured such that a first pressure in the first chamber controls the pressure applied to the substrate in the loading area, and a second pressure in the second chamber controls the size of the loading area.
Implementations of the invention may include one or more of the following features. A vertically movable base may form at least part of an upper boundary of the second pressurizable chamber. A housing may be connectable to a drive shaft and a third chamber may be disposed between the housing and the base. A retaining ring may be connected to the base to maintain the substrate beneath the carrier head. A boundary between the first and second chambers may be formed by a rigid member or a flexible member, and the second chamber may form a generally annular volume or a generally solid volume. The lower surface of the first flexible membrane may provide a mounting surface for the substrate, or a second flexible membrane may extend beneath the first flexible membrane to provide a mounting surface for the substrate. The volume between the first flexible membrane and the second flexible membrane may define a third pressurizable chamber. The first flexible membrane may be movable into contact with an upper surface of the second flexible membrane in the loading area to apply pressure to the substrate. The lower surface of the first flexible membrane may be textured to provide fluid flow between the first and second flexible membranes when they are in contact.
A first support structure may positioned inside the first chamber, and the first flexible membrane may extends around an outer surface of the first support structure. A first spacer ring may be positioned outside the first chamber, and the first flexible membrane may extend in a serpentine path between the first structure and the first spacer ring, around an inner surface of the first spacer ring, and outwardly around an upper surface of the first spacer ring. A second support structure may be located in the third chamber between the first and second flexible membranes and positioned to surround the first supports structure. A second spacer ring may be located outside the third chamber above the second support ring, and the second flexible membrane may extend in a serpentine path between the second support structure and the second spacer ring, around an inner surface of the second spacer ring, and outwardly around an upper surface of the second spacer ring.
In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a base, a first flexible membrane portion, and a second flexible membrane portion. The first flexible membrane portion extends beneath the base and defines a first pressurizable chamber, and a lower surface of the first flexible membrane portion provides a mounting surface to apply a pressure to a substrate in a loading area having a controllable size. The second flexible membrane portion couples the first flexible membrane portion to the base and defines a second pressurizable chamber so that a first pressure in the first pressurizable chamber controls the pressure applied to the substrate in the loading area, and a second pressure in the second chamber controls the size of the loading area.
In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a base, a first flexible membrane portion, a second flexible membrane portion, and a third flexible membrane portion. The first flexible membrane portion extends beneath the base to define a first pressurizable chamber, and a lower surface of the first flexible membrane provides a mounting surface for a substrate. The second flexible membrane portion extends beneath the base and defines a second pressurizable chamber, and a lower surface of the second flexible membrane contacts a top surface of the first flexible membrane in a loading area having a controllable size. The third flexible membrane portion couples the second flexible membrane portion to the base and defines a third pressurizable chamber so that a first pressure in the second pressurizable chamber controls the pressure applied to the substrate in the loading area, and a second pressure in the third chamber controls the size of the loading area.
In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a first biasing member and a second biasing member. The first biasing member includes a first pressure chamber, and a lower surface of the first pressure chamber is bounded by a flexible membrane that provides a first surface to apply a load to a substrate in a loading area having a controllable size. The second biasing member is connected to the first biasing member, and the second biasing member controls the vertical position of the first biasing member so that the second biasing member controls the size of the loading area and the first biasing member controls the pressure applied to the substrate in the loading area.
In another aspect, the invention is directed to a carrier head for chemical mechanical polishing having a flexible membrane that provides a mounting surface for a substrate, means for controlling a size of a loading area in which a load is applied to the substrate, and means for controlling a pressure applied to the substrate in the loading area.
In another aspect, the invention is directed to a method for chemical mechanical polishing a substrate. In the method, a substrate is held against a polishing pad with a carrier head, a load is applied to the substrate in a loading area with a first chamber in the carrier head, the size of the loading area is controlled with a second chamber in the carrier head, and relative motion is created between the substrate and the polishing pad.
In another aspect, the invention is directed to a method of detecting a substrate in a carrier head for a chemical mechanical polishing system. In the method, a chamber in a carrier head is connected to a pressure source. The pressure in the chamber is measured as a function of time, and the derivative of the pressure in the chamber is calculated. Whether the substrate is adjacent a substrate receiving surface in the carrier head is determined from the derivative.
Implementations of the invention may include the following features. The substrate may be indicated as present if the derivative exceeds a critical value, or absent if if the derivative does not exceed a critical value.
Advantages of the invention may include the following. Both the pressure and the loading area of the flexible membrane against the substrate may be varied to compensate for non-uniform polishing. Non-uniform polishing of the substrate is reduced, and the resulting flatness and finish of the substrate are improved.
Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.